Commit message: ecryptfs: forbid opening files without mmap handler

This prevents users from triggering a stack overflow through a recursive
invocation of pagefault handling that involves mapping procfs files into
virtual memory.

Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2016-1583
CWE: CWE-787
Vulnerability description: The ecryptfs_privileged_open function in fs/ecryptfs/kthread.c in the Linux kernel before 4.6.3 allows local users to gain privileges or cause a denial of service (stack memory consumption) via vectors involving crafted mmap calls for /proc pathnames, leading to recursive pagefault handling.

Project: linux
File: kthread.c

int ecryptfs_privileged_open(struct file **lower_file,
			     struct dentry *lower_dentry,
			     struct vfsmount *lower_mnt,
			     const struct cred *cred)
{
	struct ecryptfs_open_req req;
	int flags = O_LARGEFILE;
	int rc = 0;

	init_completion(&req.done);
	req.lower_file = lower_file;
	req.path.dentry = lower_dentry;
	req.path.mnt = lower_mnt;

	/* Corresponding dput() and mntput() are done when the
	 * lower file is fput() when all eCryptfs files for the inode are
	 * released. */
	flags |= IS_RDONLY(d_inode(lower_dentry)) ? O_RDONLY : O_RDWR;
	(*lower_file) = dentry_open(&req.path, flags, cred);
	if (!IS_ERR(*lower_file))
		goto have_file;
	if ((flags & O_ACCMODE) == O_RDONLY) {
		rc = PTR_ERR((*lower_file));
		goto out;
	}
	mutex_lock(&ecryptfs_kthread_ctl.mux);
	if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
		rc = -EIO;
		mutex_unlock(&ecryptfs_kthread_ctl.mux);
		printk(KERN_ERR "%s: We are in the middle of shutting down; "
		       "aborting privileged request to open lower file\n",
			__func__);
		goto out;
	}
	list_add_tail(&req.kthread_ctl_list, &ecryptfs_kthread_ctl.req_list);
	mutex_unlock(&ecryptfs_kthread_ctl.mux);
	wake_up(&ecryptfs_kthread_ctl.wait);
	wait_for_completion(&req.done);
	if (IS_ERR(*lower_file)) {
		rc = PTR_ERR(*lower_file);
		goto out;
	}
have_file:
	if ((*lower_file)->f_op->mmap == NULL) {
		fput(*lower_file);
		*lower_file = NULL;
		rc = -EMEDIUMTYPE;
	}
out:
	return rc;
}

Project: linux
File: kthread.c

void ecryptfs_destroy_kthread(void)
{
	struct ecryptfs_open_req *req, *tmp;

	mutex_lock(&ecryptfs_kthread_ctl.mux);
	ecryptfs_kthread_ctl.flags |= ECRYPTFS_KTHREAD_ZOMBIE;
	list_for_each_entry_safe(req, tmp, &ecryptfs_kthread_ctl.req_list,
				 kthread_ctl_list) {
		list_del(&req->kthread_ctl_list);
		*req->lower_file = ERR_PTR(-EIO);
		complete(&req->done);
	}
	mutex_unlock(&ecryptfs_kthread_ctl.mux);
	kthread_stop(ecryptfs_kthread);
	wake_up(&ecryptfs_kthread_ctl.wait);
}

Project: linux
File: kthread.c

static int ecryptfs_threadfn(void *ignored)
{
	set_freezable();
	while (1)  {
		struct ecryptfs_open_req *req;

		wait_event_freezable(
			ecryptfs_kthread_ctl.wait,
			(!list_empty(&ecryptfs_kthread_ctl.req_list)
			 || kthread_should_stop()));
		mutex_lock(&ecryptfs_kthread_ctl.mux);
		if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
			mutex_unlock(&ecryptfs_kthread_ctl.mux);
			goto out;
		}
		while (!list_empty(&ecryptfs_kthread_ctl.req_list)) {
			req = list_first_entry(&ecryptfs_kthread_ctl.req_list,
					       struct ecryptfs_open_req,
					       kthread_ctl_list);
			list_del(&req->kthread_ctl_list);
			*req->lower_file = dentry_open(&req->path,
				(O_RDWR | O_LARGEFILE), current_cred());
			complete(&req->done);
		}
		mutex_unlock(&ecryptfs_kthread_ctl.mux);
	}
out:
	return 0;
}

Project: linux
File: kthread.c

int __init ecryptfs_init_kthread(void)
{
	int rc = 0;

	mutex_init(&ecryptfs_kthread_ctl.mux);
	init_waitqueue_head(&ecryptfs_kthread_ctl.wait);
	INIT_LIST_HEAD(&ecryptfs_kthread_ctl.req_list);
	ecryptfs_kthread = kthread_run(&ecryptfs_threadfn, NULL,
				       "ecryptfs-kthread");
	if (IS_ERR(ecryptfs_kthread)) {
		rc = PTR_ERR(ecryptfs_kthread);
		printk(KERN_ERR "%s: Failed to create kernel thread; rc = [%d]"
		       "\n", __func__, rc);
	}
	return rc;
}